A signal path setting circuit similar to that claimed in the instant invention has been used for a digital recorder to make a digital recording of limited sampling frequency.
FIG. 8 shows a fundamental example of a commercially used digital recorder. A digital signal S1 is modulated in accordance with a standardized digital audio interface format (hereinafter referred to as DAIF) and then the input to a digital input terminal 1 is demodulated by a DAIF demodulator 2 and then supplied to an input point SW1(a) of a switch SWI.
An analog signal S.sub.2 is first input to an analog input terminal 6, then has its level adjusted by an input potentiometer 7, its high band component attenuated by a low pass filter 8, and is finally introduced into analog-to-digital converter means (A/D converter means) 9. The A/D converter means 9 includes a sample and hold circuit 18 and an analog-to-digital converter (hereinafter referred to as A/D converter) 19 and serves to sequentially convert analog signals sampled by predetermined sampling frequency f into digital signals and to supply them to an input point SW1(b) of the switch SW1.
The low pass filter 8 is designed to attenuate frequencies of greater than f/2 with a sharp cut-off characteristic.
The switch SW1 serves to select one of the digital signals introduced at the input points SW1(a) and SW1(b) and to supply it to modulated signal recording and playing back means 3.
The modulated signal recording and playing back means 3 includes a signal processing circuit (A) 12, a modulating circuit 13, a recording and playing back mechanism 14, a demodulating circuit 15, a signal processing circuit (B) 16 and a recording/playing back switch (hereinafter referred to as RP switch) 17.
The signal processing circuit (A) 12 serves to make a code addition and an interleave of the input digital signal for correction of the errors and to supply the resulting signal to the modulating circuit 13.
The modulating circuit 13 serves to modulate the input digital signal into a digital recording signal in accordance with an 8-10 modulation system, for example, and to supply the resulting signal to the recording and playing back mechanism 14 which has a predetermined output timing.
The recording and playing back mechanism 14 includes tape drive means and rotary head means and serves to record the digital recording signal on a magnetic tape in record mode and to play it back from the magnetic tape in playback mode. The digital signal played back in the playback mode is demodulated by the demodulating circuit 15 in accordance with an 8-10 modulation system, then has the errors corrected and is deinterleaved by the signal processing circuit (B) 16 and is introduced into the playback input point (hereinafter referred to as P input point) of the RP switch 17.
The RP switch 17 directly receives the digital signal selected by the switch SW1 at a recording input point (hereinafter referred to as R input point) and supplies the R input digital signal to a DAIF modulator 4 as well as to digital-to-analog converter means (D/A converter means) 10 when in the recording mode and supplies the P input digital signal to the DAIF modulator 4 as well as to the D/A converter means 10.
The D/A converter means 10 includes a digital-to-analog converter (hereinafter referred to as D/A converter) 20 and a low pass filter 21 to attenuate the signals with a frequency of greater than f/2 with a sharp cut-off characteristic, and serves to convert the input digital signal into an analog signal S.sub.4 and to supply it to an analog output terminal 11.
Although FIG. 8 illustrates a single circuit construction of the switch SW1 and the RP switch 17 for purposes of simplification, in practice, the number of circuits may be set in accordance with the number of signals to be switched. Also, although the recorder as shown in FIG. 8 has only one analog input terminal and one analog output terminal, it may have a plurality of input terminals and output terminals for two or four channels. In this case, the analog signals introduced at the input terminals are converted by the A/D converter means, which has a multiplexer, into digital signals including a plurality of channel informations in accordance with a predetermined format. The played-back digital signal is again divided by the D/A converter means, which has a multiplexer, into a plurality of channel analog signals which are output from respective output terminals corresponding to the input terminals.
In the digital tape recorder of FIG. 8, in the recording mode, where the RP switch 17 is set to select the R input digital signal, if the switch SW1 is set so as to select the input signal at the input point SW1(b), a digital recording signal based on the analog signal S.sub.2 input to the analog input terminal 6 is recorded on the magnetic tape, while a digital output signal or an analog output signal based on the analog signal S.sub.2 is output from the digital output terminal 5 or the analog output terminal 11, respectively.
In the recording mode, if the switch SW1 is set so as to select the input signal at the input point SW1(a), a digital recording signal based on the digital signal S.sub.1 input to the digital input terminal 1 is recorded on the magnetic tape, while the digital or analog output signal corresponding to the digital signal S.sub.1 is output from the digital output terminal 5 or the analog output terminal 11, respectively.
In the playback mode, the RP switch 17 is set to introduce the digital playback signal input to the P input point and the digital or analog output signal corresponding to the digital playback signal is output from the digital output terminal 5 or the analog output terminal 11, respectively.
In the conventional digital tape recorder, if the modulated signal recording and playing back means 3 has a sampling frequency of 48 kHz or 32 kHz, for example, for which the recording operation is possible, the sampling frequency of the digital signal S.sub.1 input to the digital input terminal 1 is also limited. Thus, if a digital signal processed by the sampling frequency of 44.1 kHz output from a peripheral audio instrument such as a compact disc player is to be recorder, the digital signal is normally converted into an analog signal within the peripheral audio instrument and the analog signal is input to the analog input terminal 6 to be recorded. It will be noted that this makes the signal more susceptible to the introduction of external noise in the course of the signal's path into the input terminal 6, and this causes the quality of sound to be poorer.